1. Field of the Invention
The invention relates to an optical lens, particularly to a fixed focus lens.
2. Description of the Related Art
Recently, mobile phones with digital cameras have been widely distributed, and a four-piece photographic lens is commonly used in a digital camera having high image quality of 5 million pixels or more. The four-piece photographic lens includes, numbered in order of location from an object side to an image side, a first lens to a fourth lens. The first lens to the fourth lens have a positive, a negative, a positive, and a positive refractive power respectively, or the first lens to the fourth lens have a positive, a negative, a positive, and a negative refractive power respectively. The second lens, the third lens, and the fourth lens are aspherical lenses made of plastic, and the first lens is an aspherical lens made of molding glass or plastic.
FIG. 1 shows a schematic diagram illustrating a conventional fixed focus lens 100. Referring to FIG. 1, the fixed focus lens 100 includes, numbered in order of location from an object side to an image side, a first lens 110 with a positive refractive power, a second lens 112 with a negative refractive power, a third lens 114 with a positive refractive power, and a fourth lens 116 with a negative refractive power. Light beams filtered by an infrared filter 118 form an image on an image plane 120. Since the first lens 110 has a comparatively large refractive power, the total length of the fixed focus lens 100 is allowed to be reduced. However, the total length is not allowed to be further reduced because of the large interval between each two adjacent lenses. Besides, the radius of curvature of a rear surface R2 of the first lens 110 is substantially equal to the radius of curvature of a front surface R1 of the second lens 112. This results in a high tolerance of eccentricity and low assembly yield.
FIG. 2 shows a schematic diagram illustrating another conventional fixed focus lens 200. Referring to FIG. 2, the fixed focus lens 200 includes, numbered in order of location from an object side to an image side, a first lens 210, a second lens 212, a third lens 214, and a fourth lens 216. The fixed focus lens 200 has restricted field of view, and the radius of curvature of a rear surface R2 of the second lens 212 is considerably small to effectively increase relative luminance. However, such configuration results in a considerably long total length of the fixed focus lens 200 to fail to provide a light, thin, short, and small micro lens. Also, since the interval between a shoulder part of the first lens 210 and a shoulder part of the second lens 212 is considerably small, the assembly of the fixed focus lens 200 is sensitive to the tolerance of eccentricity to make mass production difficult. On the other hand, in case the interval between two shoulder parts are increased to alleviate the influence of the eccentricity tolerance, the total length of the fixed focus lens 200 is also increased, and, in turn, it is much more difficult to reduce the total length of the fixed focus lens 200.